Rubber Composition for Tire Tread

ABSTRACT

A rubber composition for a tire tread is a rubber composition containing from 66 to 110 parts by weight of a filler containing not less than 50 wt. % of a silica per 100 parts by weight of a diene rubber containing not less than 40 wt. % of a terminal-modified styrene-butadiene rubber, from 8 to 35 wt. % of a natural rubber, and from 15 to 40 wt. % of a butadiene rubber; the terminal-modified styrene-butadiene rubber having a terminal functional group derived from a compound reacting with silanol groups; a styrene unit content thereof being from 38 to 48 wt. %; an oil-extended oil content thereof being less than 30 wt. %; a ratio (BR/NR) of a compounded amount of the butadiene rubber (BR) to the natural rubber (NR) being more than 1.0 and not more than 2.5; and an embrittlement temperature of the rubber composition being not more than −45° C.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/316,097, filed on Dec. 2, 2016, which is the National Stage ofInternational Patent Application No. PCT/JP2015/066091, filed on Jun. 3,2015, which claims the benefit of priority from Japan Patent ApplicationNo. 2014-115539, filed on Jun. 4, 2014.

TECHNICAL FIELD

The present technology relates to a rubber composition for a tire treadwhich enhances wet performance, wear resistance, and performance onsnow.

BACKGROUND ART

All-season pneumatic tires for passenger vehicles are required toexcellent performance on snow when running on a snow-covered roadsurface as well as wet performance or wear resistance when running on aroad surface not covered with snow (wet road surface or dry roadsurface).

In order to enhance wet performance, silica is typically blended into arubber composition for a tire. However, in comparison to carbon black,the reinforcement performance when blended into a diene rubber is small,which leads to the problem of reduced wear resistance. In addition, whenthe compounded amount of silica is increased or the particle sizethereof is made fine in order to enhance wet performance, there areproblems in that the dispersibility of the silica decreases and the wearresistance is further diminished, or the flexibility of the rubbercomposition is lost and the performance on snow is diminished.

To resolve this problem, Japanese Unexamined Patent ApplicationPublication No. 2009-91498A and International Patent ApplicationPublication No. WO/2011/105362 propose enhancing the dispersibility ofsilica with a rubber composition in which silica is compounded in aterminal-modified styrene-butadiene rubber having terminals modifiedwith a polyorganosiloxane or the like. However, the required levelanticipated to enhance wet performance and performance on snow is evenhigher, and there is a demand to further enhance these characteristics.

SUMMARY

The present technology provides a rubber composition for a tire treadwhich enhances the balance between the performance on snow and the wetperformance and wear resistance to or beyond conventional levels.

The rubber composition for a tire tread according to the presenttechnology which achieves the object described above is a rubbercomposition containing from 66 to 110 parts by weight of a fillercontaining not less than 50 wt. % of a silica per 100 parts by weight ofa diene rubber containing not less than 40 wt. % of a terminal-modifiedstyrene-butadiene rubber, from 8 to 35 wt. % of a natural rubber, andfrom 15 to 40 wt. % of a butadiene rubber; the terminal-modifiedstyrene-butadiene rubber having a terminal functional group derived froma compound that reacts with silanol groups; a styrene unit contentthereof being from 38 to 48 wt. %; an extender oil content thereof beingless than 30 wt. %; a ratio (BR/NR) of a compounded amount of thebutadiene rubber (BR) to the natural rubber (NR) being larger than 1.0and not larger than 2.5; and an embrittlement temperature of the rubbercomposition being not higher than −45° C.

The rubber composition for a tire tread according to the presenttechnology is prepared by compounding from 66 to 110 parts by weight ofa filler containing not less than 50 wt. % of a silica into 100 parts byweight of a diene rubber containing: a terminal-modifiedstyrene-butadiene rubber containing a functional group that reacts withsilanol groups on the silica surface, the styrene unit content thereofbeing from 38 to 48 wt. % and the amount of oil extension thereof beingless than 30 parts by weight; a natural rubber; and a butadiene rubber.Therefore, it is possible to enhance the wet performance and wearresistance by increasing the affinity between the diene rubber and thesilica and enhancing the dispersibility of the silica. Furthermore,since the ratio (BR/NR) of the compounded amounts of the butadienerubber (BR) and the natural rubber (NR) is larger than 1 and not largerthan 2.5 and the embrittlement temperature is set to not higher than−45° C., it is possible to enhance the balance between the performanceon snow and the wet performance and wear resistance to or beyondconventional levels.

The silica preferably has a dibutyl phthalate absorption (DBP) of 160 to220 mL/100 g, a nitrogen adsorption specific surface area of 145 to 193m²/g, and a cetyl trimethyl ammonium bromide (CTAB) specific surfacearea of from 140 to 184 m²/g. By compounding such a silica, it ispossible to not only further enhance the affinity with theterminal-modified styrene-butadiene rubber, but also to further enhancethe performance on snow.

The total amount of the oil component out of 100 wt. % of the rubbercomposition for a tire tread should be from 25 to 50 wt. %.

The terminal-modified styrene-butadiene rubber is preferably one inwhich the functional groups of one or both terminals thereof are derivedfrom at least one type of compound selected from the group consisting ofpolyorganosiloxane compounds, epoxy compounds, and hydrocarbyloxysilicon compounds. In addition, the terminal-modified styrene-butadienerubber may have an isoprene segment in one of the terminals thereof.

The balance between the performance on snow and the wet performance andwear resistance can be enhanced to or beyond conventional levels with apneumatic tire in which the rubber composition of the present technologyis used in the tread portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view in a tire meridian directionillustrating an example of an embodiment of a pneumatic tire in which arubber composition for a tire tread according to the present technologyis used.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of an embodiment of a pneumatic tire inwhich a rubber composition for a tire tread is used. The pneumatic tireincludes a tread portion 1, sidewall portions 2, and bead portions 3.

In FIG. 1, in a pneumatic tire, two layers of a carcass layer 4, formedby arranging reinforcing cords, which extend in a tire radial direction,in a tire circumferential direction at a predetermined pitch andembedding the reinforcing cords in a rubber layer, are disposedextending between the left and right side bead portions 3. Both ends ofthe carcass layer 4 are made to sandwich a bead filler 6 around a beadcore 5 that is embedded in the bead portions 3 and are folded back in atire axial direction from the inside to the outside. An innerliner layer7 is disposed inward of the carcass layer 4. Two layers of a belt layer8, formed by arranging reinforcing cords, which extend inclined in thetire circumferential direction, in the tire axial direction at apredetermined pitch and embedding these reinforcing cords in a rubberlayer, are disposed on an outer circumferential side of the carcasslayer 4 of the tread portion 1. The reinforcing cords of the two layersof the belt layer 8 intersect interlaminarly so that the directions ofinclination with respect to the tire circumferential direction areopposite each other. The belt cover layer 9 is disposed on an outercircumferential side of the belt layer 8. The tread portion 1 is formedfrom a tread rubber layer 10 on an outer circumferential side of thebelt cover layer 9. The tread rubber layer 10 is preferably composed ofthe rubber composition for a tire tread of the present technology. Aside rubber layer 11 is disposed outward of the carcass layer 4 in eachside wall portion 2, and a rim cushion rubber layer 12 is providedoutward of the portion of the carcass layer 4 that is folded back aroundeach of the bead portions 3. It should be noted that a studless tire isnot limited to an embodiment of the pneumatic tire illustrated in FIG. 1as an example.

In the rubber composition for a tire tread according to the presenttechnology, the rubber component is a diene rubber, and the diene rubbernecessarily contains a terminal-modified styrene-butadiene rubber, anatural rubber, and a butadiene rubber. The terminal-modifiedstyrene-butadiene rubber is a styrene-butadiene rubber produced bysolution polymerization so as to have functional groups at one or bothterminals of the molecular chain. By compounding the terminal-modifiedstyrene-butadiene rubber, the affinity with silica is increased, and thedispersibility is improved. As a result, the effects of the silica arefurther enhanced and the wear resistance is improved.

In the present technology, the backbone of the modifiedstyrene-butadiene rubber preferably has an isoprene segment at one ofthe terminals thereof. By configuring one of the terminals of thestyrene-butadiene rubber with an isoprene segment, the affinity withsilica is enhanced, and the wet performance and wear resistance arefurther improved.

In addition, the functional group of the modified styrene-butadienerubber is a functional group derived from a compound that reacts withthe silanol groups on the silica surface. The compound that reacts withsilanol groups is not particularly limited, but examples thereof includepolyorganosiloxane compounds, epoxy compounds, hydrocarbyloxy siliconcompounds, tin compounds, silicon compounds, silane compounds, amidecompounds and/or imide compounds, isocyanate and/or isothiocyanatecompounds, ketone compounds, ester compounds, vinyl compounds, oxysilanecompounds, thiirane compounds, oxetane compounds, polysulfide compounds,polysiloxane compounds, polyether compounds, polyene compounds, halogencompounds, and compounds containing fullerenes or the like. Among these,polyorganosiloxane compounds, epoxy compounds, and hydrocarbyloxysilicon compounds are preferable.

In the terminal-modified styrene-butadiene rubber that is used in thepresent technology, the styrene unit content is from 38 to 48 wt. % andpreferably from 40 to 45 wt. %. By setting the styrene unit content ofthe terminal-modified styrene-butadiene rubber to within such a range,it is possible to increase the rigidity and strength of the rubbercomposition and to further increase the wear resistance and wetperformance when formed into a pneumatic tire. In addition, when a dienerubber other than a butadiene rubber is compounded, theterminal-modified styrene-butadiene rubber takes on a finephase-separated form from the other diene rubber. As a result, theterminal-modified styrene-butadiene rubber gathers locally in thevicinity of the silica particles, and the terminal modified groups acteffectively on the silica, which makes it possible to further enhanceaffinity and to improve the dispersibility of the silica. If the styreneunit content of the terminal-modified styrene-butadiene rubber is lessthan 38 wt. %, the effect of forming a fine phase-separated form fromthe other diene cannot be sufficiently achieved. Additionally, theeffects of increasing the rigidity and the strength of the rubbercomposition cannot be sufficiently obtained. Furthermore, if the styreneunit content of the terminal-modified styrene-butadiene rubber exceeds48 wt. %, the glass transition temperature (Tg) of the styrene-butadienerubber increases, and the balance of the viscoelastic characteristicsbecomes poor. Note that the styrene unit content of theterminal-modified styrene-butadiene rubber is measured by infraredspectroscopy (Hampton method).

In the present technology, the concentration of terminal-modified groupin the terminal-modified styrene-butadiene rubber is determined by therelationship to the weight average molecular weight (Mw) of theterminal-modified styrene-butadiene rubber. The weight average molecularweight of the terminal-modified styrene-butadiene rubber is preferablyfrom 600,000 to 1,000,000 and more preferably from 650,000 to 850,000.If the weight average molecular weight of the terminal-modifiedstyrene-butadiene rubber is less than 600,000, the modified groupconcentration of the terminal-modified styrene-butadiene rubber becomeshigh and the characteristics of the rubber composition (for example, thesilica dispersibility) are better, but since the molecular weight of thepolymer itself is low, there is a possibility that the strength andrigidity will be insufficient, and the degree of improvement in theviscoelastic characteristics at high temperatures is small. Furthermore,the wear resistance of the rubber composition may decline. In addition,if the weight average molecular weight of the terminal-modifiedstyrene-butadiene rubber exceeds 1,000,000, the modified groupconcentration of the terminal-modified styrene-butadiene rubber becomeslow, and the affinity with silica becomes insufficient, which leads tothe risk that the dispersibility may become poor. Note that the weightaverage molecular weight (Mw) of the terminal-modified styrene-butadienerubber is measured by gel permeation chromatography (GPC) on the basisof a polystyrene standard.

In the present technology, the vinyl unit content of theterminal-modified styrene-butadiene rubber is preferably from 20 to 35wt. % and more preferably from 26 to 34 wt. %. By setting the vinyl unitcontent of the terminal-modified styrene-butadiene rubber to 20 to 35wt. %, it is possible to optimize the glass transition temperature (Tg)of the terminal-modified styrene-butadiene rubber. In addition, it ispossible to stabilize the fine phase-separated form of theterminal-modified styrene-butadiene rubber formed with respect to theother diene rubber. If the vinyl unit content of the terminal-modifiedstyrene-butadiene rubber is less than 20 wt. %, there is a risk that theTg of the terminal-modified styrene-butadiene rubber may become low andthat the wet grip performance may be diminished. In addition, if thevinyl unit content of the terminal-modified styrene-butadiene rubberexceeds 35 wt. %, there is a possibility that the strength or rigiditymay decrease and that the loss tangent (tan δ at 60° C.) may becomelarge. Note that the vinyl unit content of the terminal-modifiedstyrene-butadiene rubber is measured by infrared spectroscopy (Hamptonmethod).

The forming processability of the rubber composition can be enhanced byadding an oil component (oil-extension) to the terminal-modifiedstyrene-butadiene rubber. The amount of oil extension is less than 30wt. % and preferably not less than 10 wt. % and less than 30 wt. % outof 100 wt. % of the terminal-modified styrene-butadiene rubber. If theamount of oil extension of the terminal-modified styrene-butadienerubber exceeds 30 parts by weight, the degree of freedom in compositiondesign when compounding oils, softeners, tackifiers, and the like in therubber composition will become small.

In the present technology, the content of the terminal-modifiedstyrene-butadiene rubber is not less than 40 wt. %, preferably from 40to 78 wt. %, more preferably from 42 to 70 wt. %, and even morepreferably from 45 to 60 wt. % out of 100 wt. % of the diene rubber. Ifthe content of the terminal-modified styrene-butadiene rubber is lessthan 40 wt. % in the diene rubber, the affinity with the silica willdecline, so the dispersibility thereof cannot be enhanced. If thecontent of the terminal-modified styrene-butadiene rubber exceeds 78 wt.% in the diene rubber, there is a risk that the wear resistance maydecline.

Since the rubber composition for a tire tread according to the presenttechnology contains a natural rubber, it is possible to enhance the wearresistance and wet grip performance while maintaining a high level ofperformance on snow. The compounded amount of the natural rubber is from8 to 35 wt. % and preferably from 10 to 25 wt. % in 100 wt. % of thediene rubber. If the compounded amount of the natural rubber is lessthan 8 wt. %, the performance on snow, wet grip performance, and wearresistance cannot be sufficiently enhanced. Additionally, if thecompounded amount of the natural rubber exceeds 35 weight %, the wetgrip performance will decline. A natural rubber that is regularly usedin rubber compositions for a tire is preferably used.

Since the rubber composition for a tire tread according to the presenttechnology contains a butadiene rubber, it is possible to enhance thewear resistance and performance on snow. The compounded amount of thebutadiene rubber is from 15 to 40 wt. % and preferably from 25 to 35 wt.% in 100 wt. % of the diene rubber. If the compounded amount of thebutadiene rubber is less than 8 wt. %, the wear resistance will decline.Additionally, if the compounded amount of the butadiene rubber exceeds40 wt. %, there is a concern that tipping resistance may be diminished.Any butadiene rubber that is regularly used in rubber compositions for atire may be used.

In the present technology, the compounded amount ratio (BR/NR) of thecompounded amount of the butadiene rubber (BR) to the compounded amountof the natural rubber (NR) is larger than 1.0 and not larger than 2.5and preferably from 1.5 to 2.3. If the ratio (BR/NR) of the compoundedamounts of the butadiene rubber and the natural rubber is not largerthan 1.0, it is not possible to enhance the balance between theperformance on snow and the wet performance and wear resistance, and itis not possible to enhance the performance on snow, in particular. Inaddition, if the ratio (BR/NR) of the compounded amounts exceeds 2.5,the wet grip performance will decline.

In the present technology, diene rubbers other than a terminal-modifiedstyrene-butadiene rubber, a natural rubber, and a butadiene rubber maybe compounded as diene rubbers within a range that does not diminish thepurpose of the present technology. Examples of other diene rubbersinclude isoprene rubber, unmodified styrenebutadiene rubber, butylrubber, and halogenated butyl rubber. A single rubber may be used ormultiple rubbers may be blended and used as the diene rubber.

The rubber composition for a tire tread according to the presenttechnology contains from 66 to 110 parts by weight of a fillercontaining not less than 50 wt. % of a silica per 100 parts by weight ofthe diene rubber. By setting the compounded amount of the filler towithin such a range, it is possible to balance the wet grip performanceand the wear resistance of the rubber composition at a higher level. Ifthe compounded amount of the filler is less than 66 parts by weight, ahigh level of wet grip performance cannot be secured. If the compoundedamount of the filler exceeds 110 parts by weight the wear resistancewill be diminished.

The content of the silica in 100 wt. % of the filler is not less than 50wt. % and preferably from 70 to 100 wt. %. By setting the content of thesilica in the filler to within such a range, it is possible to achieveboth the wet grip performance and the wear resistance of the rubbercomposition. In addition, by compounding the terminal-modifiedstyrene-butadiene rubber, the affinity with the silica is increased andthe dispersibility is enhanced, which makes it possible to furtherenhance the effect of compounding silica.

The silica may be any silica that is regularly used in rubbercompositions for a tire tread. Examples thereof include wet methodsilica, dry method silica, surface treated silica, and the like.Additionally, the particle characteristics of the silica are notparticularly limited but preferably satisfy all three of the particlecharacteristics below.

(1) DBP Absorption Number: From 160 to 220 mL/100 g

The DBP absorption number of the silica is preferably set to be from 160to 220 mL/100 g. If the DBP absorption number is less than 160 mL/100 g,breaking strength will decline. If the DBP absorption number exceeds 220mL/100 g, viscosity will excessively increase and mixing processabilitywill be negatively affected. The DBP absorption number of the silica iscalculated in accordance with Oil Absorption Number Method A describedin JIS (Japanese Industrial Standard) K6217-4.

(2) Nitrogen Specific Surface Area (N₂SA): From 145 to 193 m²/g

The nitrogen specific surface area (N₂SA) of the silica is preferablyset to be from 145 to 193 m²/g. It is not preferable for the N₂SA of thesilica to be less than 145 m²/g because the wet grip performance will bediminished. If the N₂SA of the silica exceeds 193 m²/g, thedispersibility of the silica will decline, and the wear resistance willbe diminished. In addition, the rubber composition will become hard, andthe performance on snow will decline, which is not preferable. The N₂SAof the silica is calculated in accordance with JIS K6217-2.

(3) CTAB Specific Surface Area (CTAB): From 140 to 184 m²/g

The CTAB specific surface area (CTAB) of the silica is preferably set tobe from 140 to 184 m²/g. It is not preferable for the CTAB of the silicato be less than 140 m²/g because the wet grip performance will bediminished. It is also not preferable for the CTAB of the silica toexceed 184 m²/g because the dispersibility of the silica will declineand the wear resistance will be diminished. The CTAB of the silica iscalculated in accordance with JIS K6217-3.

The dispersibility of the silica can be enhanced by compounding a silicasatisfying all of the particle characteristics of (1) to (3) describedabove together with the terminal-modified styrene-butadiene rubberdescribed above. Therefore, the terminal-modified styrene-butadienerubber and the silica having the particle characteristics describedabove both act to modify the tan δ of the rubber composition, whichmakes it possible to achieve even greater synergy. In addition, the wearresistance and performance on snow of the rubber composition can be madeexcellent by compounding both a natural rubber and a butadiene rubberinto the terminal-modified styrene-butadiene rubber.

In the present technology, a silica satisfying all of the particlecharacteristics of (1) to (3) may be used alone as a silica.Alternatively, this silica may also be used together with another silicathat does not satisfy the particle characteristics of (1) to (3).

The silica to be used may be appropriately selected from commerciallyavailable products. Additionally, a silica obtained through a regularmanufacturing method may be used.

In the rubber composition of the present technology, a silane couplingagent is preferably compounded together with the silica as such willlead to an improvement in the dispersibility of the silica and a furtherincrease in the reinforcement action of the diene rubber. The blendingquantity of the silane coupling agent is preferably from 3 to 20 wt. %,and more preferably from 5 to 15 wt. %, of the blending quantity of thesilica. If the compounded amount of the silane coupling agent is lessthan 3 wt. % of the weight of the silica, the effect of improving thedispersibility of the silica cannot be sufficiently obtained.Furthermore, when the compounded amount of the silane coupling agentexceeds 20 wt. %, the silane coupling agents condense with each other,and the desired effects cannot be obtained.

The silane coupling agent is not particularly limited, but is preferablya sulfur-containing silane coupling agent. Examples thereof includebis-(3-triethoxysilylpropyl)tetrasulfide,bis(3-triethoxysilylpropyl)disulfide, 3-trimethoxysilylpropylbenzothiazole tetrasulfide, γ-mercaptopropyltriethoxysilane, and3-octanoylthiopropyl triethoxysilane.

The rubber composition for a tire tread of the present technology mayalso include fillers other than the silica. Examples of such fillersother than the silica include, carbon black, clay, mica, talc, calciumcarbonate, aluminum hydroxide, aluminum oxide, and titanium oxide. Amongthese, carbon black is preferable. This is because rubber strength canbe increased by compounding other fillers, including carbon black. Acontent of the other fillers is not greater than 50 wt. % and ispreferably from 0% to 30 wt. % of 100 wt. % of the filler. If thecontent of the other fillers exceeds 50 wt. %, the rolling resistancewill worsen.

The embrittlement temperature of the rubber composition for a tire treadaccording to the present technology is not higher than −45° C., and ispreferably from −60° C. to −50° C. If the embrittlement temperature ofthe rubber composition is higher than −45° C., the performance on snowwill be diminished. The embrittlement temperature of the rubbercomposition for a tire tread is the 50% impact embrittlement temperaturedetermined in accordance with JIS K6261.

The total amount of the oil components in the rubber composition for atire tread should be from 25 to 50 wt. % and more preferably from 30 to45 wt. % out of 100 wt. % of the rubber composition. If the total amountof the oil components is less than 25 wt. %, there is a risk that it maynot be possible to sufficiently enhance the performance on snow. Inaddition, if the total amount of the oil components exceeds 50 wt. %,there is a risk that it may not be possible to sufficiently enhance thewear resistance. Note that the total amount of the oil components refersto the oil components contained in the rubber composition composed ofoil components such as an extender oil in the diene rubber as well as anatural oil, a synthetic oil, a plasticizer, and the like added in acase of the preparation of the rubber composition.

The rubber composition for a tire tread may also contain variouscompounding agents that are commonly used in rubber compositions for atire tread. Examples thereof include vulcanization or cross-linkingagents, vulcanization accelerators, antiaging agents, plasticizers,processing aids, liquid polymers, terpene resins, and thermosettingresins. These compounding agents can be kneaded by a common method toobtain a rubber composition that can then be used for vulcanization orcross-linking. These compounding agents can be compounded in typicalamounts conventionally used so long as the objects of the presenttechnology are not hindered. The rubber composition for a tire tread canbe produced by mixing the above-mentioned components using a well-knownrubber kneading machine such as a Banbury mixer, a kneader, a roller, orthe like.

The rubber composition for a tire tread of the present technology can beadvantageously used in pneumatic tires. The balance between theperformance on snow when running on a snow-covered road surface and thewet performance or wear resistance when running on a road surface notcovered with snow can be enhanced to or beyond conventional levels witha pneumatic tire in which this rubber composition is used in a tiretread portion and, in particular, an all-season pneumatic tire for apassenger vehicle.

The present technology is further described below using examples.

However, the scope of the present technology is not limited to theseexamples.

Examples

Fifteen types of rubber compositions for a tire tread (Working Examples1 to 7 and Comparative Examples 1 to 8) were prepared according to theformulations shown in Tables 1 and 2 with the compounding agents shownin Table 3 used as common components. With the exception of the sulfurand the vulcanization accelerators, the components were measured andkneaded in a 1.8 L sealed mixer for 5 minutes, and the master batch wasthis discharged and cooled at room temperature. The master batch was fedto a 1.8 L kneader, and the sulfur and the vulcanization acceleratorwere added to the kneader and mixed to obtain a rubber composition for atire tread. Note that it Tables 1 and 2, the modified S-SBR 1 and 2 areoil extended products, so the net rubber amounts are also included inparentheses. In addition, the compounded amounts of the commoncomponents in Table 3 are listed as the parts by weight per 100 parts byweight of the diene rubber shown in Tables 1 and 2.

Pneumatic tires of a size used in cap treads (225/60R18) werevulcanization-molded using the 15 types of rubber compositions that wereobtained. The wear resistance, performance on snow, and wet performancewere evaluated by the methods described below using each of thepneumatic tires.

Wear Resistance

The obtained pneumatic tires were assembled on wheels with a rim size of18×7 JJ, filled to an air pressure of 220 kPa, and mounted on a 2.5 Lclass test vehicle (made in Japan). The vehicle was driven ten laps at aspeed of 80 km/h over a dry road surface of a test course 5 km in lengthper lap. The state of wear on the tread surface was then observedvisually and evaluated with a score using Comparative Example 1 as 100.The results were recorded in Tables 1 and 2. Larger evaluation values,particularly index values of 102 or greater, indicate superior wearresistance.

Performance on Snow

The obtained pneumatic tires were assembled on wheels having a rim sizeof 18×7JJ and mounted on a 2.5 L class test vehicle (made in Japan). Thevehicle was driven on a test course of 2.6 km per lap in a snow-coveredstate under conditions with an air pressure of 200 kPa, and the steeringstability at that time was scored based on sensory evaluation of threeexperienced evaluators. The obtained results were indexed and recordedin Tables 1 to 2, with the index value of Comparative Example 1 being100. Larger index values, particularly index values of 102 and above,indicate superior performance on snow (steering stability) onsnow-covered road surfaces.

Wet Performance

The obtained pneumatic tires were assembled on wheels having a rim sizeof 18×7JJ and mounted on a 2.5 L class test vehicle (made in Japan). Thevehicle was driven on a test course of 2.6 km per lap with a wet roadsurface under conditions with an air pressure of 220 kPa, and thesteering stability at that time was scored based on sensory evaluationof three experienced evaluators. The obtained results were indexed andrecorded in Tables 1 to 2, with the index value of Comparative Example 1being 100. Larger index values, particularly index values of 102 andabove, indicate superior wet steering stability on wet road surfaces.

TABLE 1-1 Comparative Comparative Comparative Comparative Example 1Example 2 Example 3 Example 4 Modified S-SBR 1 Part by weight 61.87 (45)61.87 (45) 103.12 (75) 38.50 (28) BR Part by weight 25 40 15 40 NR Partby weight 30 15 10 32 Silica 1 Part by weight 60 60 60 60 Carbon blackPart by weight 20 20 20 20 Coupling agent Part by weight 4.0 4.0 4.0 4.0Oil Part by weight 20 20 0 26.5 Weight ratio (BR/NR) — 0.83 2.67 1.501.25 Silica ratio wt. % 75 75 75 75 Total filler (Part by weight) 80 8080 80 Total oil components (Part by weight) 36.9 36.9 28.1 37.0Embrittlement temperature ° C. −40 −53 −38 −58 Wear resistance Indexvalue 100 120 95 122 Performance on snow Index value 100 108 90 112 Wetperformance Index value 100 95 108 90

TABLE 1-2 Comparative Comparative Comparative Comparative Example 5Example 6 Example 7 Example 8 Modified S-SBR 1 Part by weight 56.65(41.2) 61.87 (45) 61.87 (45) 61.87 (45) BR Part by weight 42 35 35 35 NRPart by weight 16.8 20 20 20 Silica 1 Part by weight 60 35 45 80 Carbonblack Part by weight 20 45 20 35 Coupling agent Part by weight 4.0 2.53.2 4.0 Oil Part by weight 22 20 20 20 Weight ratio (BR/NR) — 2.50 1.751.75 1.75 Silica ratio wt. % 75 44 69 70 Total filler (Part by weight)80 80 65 115 Total oil components (Part by weight) 37.0 36.9 36.9 36.9Embrittlement temperature ° C. −55 −47 −48 −47 Wear resistance Indexvalue 125 118 121 99 Performance on snow Index value 110 109 111 96 Wetperformance Index value 93 92 94 110

TABLE 2-1 Working Working Working Working Example 1 Example 2 Example 3Example 4 Modified S-SBR 1 Part by weight 61.87 (45) 61.87 (45) 61.87(45) 61.87 (45) Modified S-SBR 2 Part by weight BR Part by weight 35 3535 35 NR Part by weight 20 20 20 20 Silica 1 Part by weight 60 60 60Silica 2 Part by weight 60 Carbon black Part by weight 20 20 20 20Coupling agent Part by weight 4.0 4.0 4.0 4.0 Oil Part by weight 5 35 2020 Weight ratio (BR/NR) — 1.75 1.75 1.75 1.75 Silica proportion wt. % 7575 0 75 Total filler (Part by weight) 80 80 80 80 Total oil components(Part by weight) 21.9 51.9 36.9 36.9 Embrittlement temperature ° C. −48−51 −50 −50 Wear resistance Index value 110 102 102 105 Performance onsnow Index value 101 120 105 115 Wet performance Index value 105 103 104104

TABLE 2-2 Working Working Working Example 5 Example 6 Example 7 ModifiedPart by 68.75 (50) 89.37 (65) S-SBR 1 weight Modified Part by 61.87 (45)S-SBR 2 weight BR Part by 35 20 35 weight NR Part by 15 15 20 weightSilica 1 Part by 60 60 60 weight Silica 2 Part by weight Carbon blackPart by 20 20 20 weight Coupling Part by 4.0 4.0 4.0 agent weight OilPart by 18.2 12.6 20 weight Weight ratio — 2.33 1.33 1.75 (BR/NR) Silicawt. % 75 75 75 proportion Total filler (Part by 80 80 80 weight) Totaloil (Part by 37.0 37.0 36.9 components weight) Embrittlement ° C. −47−45 −53 temperature Wear Index 102 101 105 resistance value PerformanceIndex 113 103 117 on snow value Wet Index 106 107 103 performance value

The types of raw materials used as indicated in Tables 1 to 2 aredescribed below.

-   -   Modified S-SBR 1: Terminal-modified solution polymerization        styrene-butadiene rubber prepared according to the production        method described below; terminal-modified solution-polymerized        styrene-butadiene rubber; styrene unit content of 42 wt. %;        vinyl unit content of 32 wt. %; weight average molecular weight        (Mw) of 750,000; Tg of −25° C.; extender oil content of 27.27        wt. %.    -   Modified S-SBR 2: Terminal-modified solution-polymerized        styrene-butadiene rubber prepared according to the production        method described below; terminal-modified styrene-butadiene        rubber; styrene unit content of 21 wt. %; vinyl unit content of        63 wt. %; weight average molecular weight (Mw) of 440,000; Tg of        −27° C.; extender oil content of 27.27 wt. %.    -   BR: Butadiene rubber; Nipol BR1220, manufactured by Zeon        Corporation    -   NR: natural rubber; SIR20    -   Silica 1: Zeosil 1165MP, manufactured by Rhodia; DBP absorption        number of 200 mL/100 g; nitrogen specific surface area (N₂SA) of        160 m²/g; CTAB specific surface area (CTAB) of 159 m²/g.    -   Silica 2: Zeosil Premium 200 MP, manufactured by Rhodia; DBP        absorption number of 203 mL/100 g; nitrogen specific surface        area (N₂SA) of 200 m²/g; CTAB specific surface area (CTAB) of        197 m²/g.    -   Carbon black: SEAST KH, manufactured by Tokai Carbon Co., Ltd.    -   Silane coupling agent: Bis(3-triethoxysilylpropyl)tetrasulfide;        Si69, manufactured by Evonik-Degussa    -   Oil: Extract No. 4S, manufactured by Showa Shell Sekiyu K.K.

Production Method of Modified S-SBR 1

To a nitrogen-purged autoclave reaction vessel having an internalcapacity of 10, 4533 g of cyclohexane, 338.9 g (3.254 mol) of styrene,468.0 g (8.652 mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and0.189 mL (1.271 mmol) of N,N,N′,N′-tetramethylethylenediamine were addedL. Then, agitation was begun. After the temperature of the content inthe reaction vessel was adjusted to 50° C., 5.061 mL (7.945 mmol) ofn-butyllithium was added. After the polymerization conversion ratereached approximately 100%, 12.0 g of isoprene was added and the mixturewas reacted for 5 minutes. Then, 0.281 g (0.318 mmol) of a toluenesolution containing 40 wt. % of 1,6-bis(trichlorosilyl)hexane was addedand the mixture was reacted for 30 minutes. Furthermore, 18.3 g (0.318mmol) of a xylene solution containing 40 wt. % of polyorganosiloxane Adescribed below was added and the mixture was reacted for 30 minutes.Then, 0.5 mL of methanol was added and the mixture was agitated for 30minutes. A small amount of antiaging agent (IRGANOX 1520, manufacturedby BASF) was added to the resulting polymer solution, and after FukkoLuella Ceramic 30 (manufactured by Nippon Oil Corporation) was added sothat the extender oil content was 27.27 wt. %, the solid rubber wasrecovered by a steam stripping method. The obtained solid rubber wasdehydrated using a roll and dried in a dryer. Thus, the modified S-SBR 1was obtained.

Polyorganosiloxane A: Polyorganosiloxane having the structure of generalformula (1): where m=80, k=120, X₁, X₄, R₁ to R₃, and R₅ to R₈ are eachmethyl groups (—CH₃), and X₂ is a hydrocarbon group expressed by generalformula (2) below.

Production Method of Modified S-SBR 2

To a nitrogen-purged 100 mL ampoule bottle, 28 g of cyclohexane and 8.6mmol of tetramethylethylenediamine were added, and then 6.1 mmol ofn-butyllithium was further added. Then, 8.0 g of isoprene was slowlyadded, and the mixture was reacted for 120 minutes in the 60° C. ampoulebottle to yield isoprene block (used as initiator 1). This initiator 1had a weight average molecular weight (Mw) of 2,200, a molecular weightdistribution (Mw/Mn) of 1.08, and an isoprene unit-derived vinyl bondcontent of 72.3 wt. %.

Then, in a nitrogen atmosphere in an autoclave equipped with a stirrer,4,000 g of cyclohexane, 357.7 g of 1,3-butadiene, and 132.3 g of styrenewere loaded, and then the entire amount of initiator 1 was added, andpolymerization of the mixture was initiated at 40° C. Ten minutes afterpolymerization was initiated, 195.3 g of 1,3-butadiene and 14.7 g ofstyrene were continuously added over the course of 60 minutes. Themaximum temperature during the polymerization reaction was 60° C. Aftercontinuous addition was completed, the polymerization reaction wascontinued for another 20 minutes, and after it was confirmed that thepolymer conversion rate had reached from 95 to 100%, 0.08 mmol of1,6-bis(trichlorosilyl)hexane was added in the state of a cyclohexanesolution having a 20 mass % concentration, and the mixture was reactedfor 10 minutes. Furthermore, 0.027 mmol of the polyorganosiloxane Arepresented by Formula (1) below was added in the state of a xylenesolution having a 20 mass % concentration, and the mixture was reactedfor 30 minutes. Next, methanol was added as a polymerization quencher inan amount equivalent to twice the number of moles of n-butyllithiumused, and a solution containing modified S-SBR 2 was obtained. AfterIrganox 1520L (manufactured by Ciba Specialty Chemicals Corp.) was addedto this solution as an anti-aging agent in an amount of 0.15 parts bymass per 100 parts by mass of the modified S-SBR 2, Fukko Luella Ceramic30 (manufactured by Nippon Oil Corporation) was added so that theextender oil content was 27.27 wt. %. The solvent was removed by steamstripping, after the resulting substance was vacuum-dried for 24 hoursat 60° C., a solid rubber was recovered. The obtained solid rubber wasdehydrated using a roll and dried in a dryer. Thus, the modified S-SBR 2was obtained.

TABLE 3 Formulation of common compounding agents Zinc oxide 3.0 Parts byweight Stearic acid 2.0 Parts by weight Anti-aging agent 2.0 Parts byweight Wax 2.0 Parts by weight Sulfur 2.0 Parts by weight Vulcanizationaccelerator 1 2.0 Parts by weight Vulcanization accelerator 2 1.5 Partsby weight

The types of raw materials used as in Table 3 are described below.

-   -   Zinc oxide: type III Zinc Oxide, manufactured by Seido Chemical        Industry Co., Ltd.    -   Stearic acid: beads stearic acid, manufactured by Chiba Fatty        Acid    -   Anti-aging agent: Antigen 6C, manufactured by Sumitomo Chemical        Co., Ltd.    -   Wax: SANNOC, manufactured by Ouchi Shinko Chemical Industrial    -   Sulfur: Golden Flower oil treated sulfur powder, manufactured by        Tsurumi Chemical Industry Co., Ltd.    -   Vulcanization accelerator 1: Vulcanization accelerator CBS;        Nocceler CZ-G, manufactured by Ouchi Shinko Chemical Industrial        Co., Ltd.    -   Vulcanization Accelerator 2: Vulcanization accelerator DPG;        Nocceler D, manufactured by Ouchi Shinko Chemical Industrial        Co., Ltd.

As is clear from Table 2, the balance between the performance on snowand the wet performance and wear resistance can be enhanced to or beyondconventional levels with the rubber compositions for a tire tread ofWorking Examples 1 to 8.

The rubber composition of Comparative Example 1 has a weight ratio(BR/NR) of not greater than 1, and this is used as a reference.

The weight ratio (BR/NR) of the rubber composition of ComparativeExample 2 exceeds 2.5, so the wet performance is diminished.

The rubber composition of Comparative Example 3 has an embrittlementtemperature higher than −45° C., so the performance on snow and wearresistance are diminished.

The rubber composition of Comparative Example 4 has a modified S-SBR 1content of less than 40 wt. %, so the wet performance is diminished.

The rubber composition of Comparative Example 5 has a compounded amountof butadiene rubber exceeding 40 wt. %, so the wet performance isdiminished.

The rubber composition of Comparative Example 6 has a silica weightratio of less than 50 wt. % in the filler, so the wet performance isdiminished.

The rubber composition of Comparative Example 7 has a filler compoundedamount of less than 66 parts by weight, so the wet performance isdiminished.

The rubber composition of Comparative Example 8 has a filler compoundedamount exceeding 110 parts by weight, so the performance on snow isdiminished.

1. A rubber composition for a tire tread, the composition comprisingfrom 66 to 110 parts by weight of a filler containing not less than 50wt. % of a silica per 100 parts by weight of a diene rubber containingfrom 40 to less than 60 wt. % of a terminal-modified styrene-butadienerubber, from 8 to 35 wt. % of a natural rubber, and from 15 to 40 wt. %of a butadiene rubber; the terminal-modified styrene-butadiene rubberhaving a terminal functional group derived from a compound reacting withsilanol groups; a styrene unit content thereof being from 38 to 48 wt.%; an extender oil content thereof being less than 30 wt. %; a ratio(BR/NR) of a compounded amount of the butadiene rubber (BR) to thenatural rubber (NR) being within a range of from 1.33 to 2.5; and anembrittlement temperature of the rubber composition being not higherthan −45° C.; wherein the functional group of at least one terminal ofthe terminal-modified styrene-butadiene rubber is derived from at leastone type of compound selected from the group consisting ofpolyorganosiloxane compounds and hydrocarbyloxy silicon compounds. 2.The rubber composition for a tire tread according to claim 1, wherein atotal of oil components is from 25 to 50 wt. % out of 100 wt. % of therubber composition for a tire tread.
 3. The rubber composition for atire tread according to claim 1, wherein the composition has an isoprenesegment on at least one terminal of the terminal-modifiedstyrene-butadiene rubber.
 4. A pneumatic wherein the rubber compositionfor a tire tread described in claim 1 is used.
 5. The rubber compositionfor a tire tread according to claim 1, comprising from 82 to 110 partsby weight of the filler.
 6. The rubber composition for a tire treadaccording to claim 1, comprising from 40 to 59 wt. % of theterminal-modified styrene-butadiene rubber.